The present invention relates to a process for coating a ceramic honeycomb body with a suspension.
Ceramic bodies in the form of honeycombs are used in large numbers in the field of catalysisxe2x80x94especially in the field of the catalytic purification of motor vehicle exhaust gasesxe2x80x94as carriers for catalytically active coatings. The carriers have a generally cylindrical shape and are delimited by two flat end-faces and one jacket. Channels parallel to the axis, or flow channels, through which the exhaust gas to be purified is passed, run through said carriers from one flat end-face to the other. Such carriers are also referred to as honeycomb bodies.
The catalyst for converting the pollutants present in the exhaust gas (mainly hydrocarbons, carbon monoxide and nitrogen oxides) generally consists of pulverulent materials of large surface area onto which the actual catalytically active components are deposited in highly disperse form. This catalyst is applied to the separating walls between the flow channels in the form of a coating. To coat the separating walls, or channel walls, with the pulverulent materials, the first step is to prepare a coating suspension. This is usually done by suspending the pulverulent materials in water. Depending on the particular application, the solids content (dry mass of the pulverulent materials) of the suspension is conventionally between 30 and 60 wt. %, based on the total weight of the coating suspension.
Within the framework of the present invention, the application of the coating suspension to the inner surfaces of the flow channels is referred to as coating of the honeycomb body. The processes employed for this purpose, such as dipping, pouring or pumping of the suspension, are known to those skilled in the art. Patent documents GB 1,515,733, U.S. Pat. No. 4,208,454, DE 40 40 150 C2 and DE 198 10 260 A1 may be mentioned here by way of example. After application, the coating is dried and calcined.
The requisite coating concentration (dry mass of coating material per liter of honeycomb body volume [g/l]) depends on the particular application and is typically between 50 and 400 g/l. To avoid high production costs, attempts are made to apply the required amount of coating in one operation, making it necessary to use correspondingly highly concentrated coating suspensions.
A substantial problem with coating is the possible obstruction of the flow channels with excess coating material as a result of the suction capacity of the porous ceramic honeycomb bodies for the liquid phase of the coating suspension, and the consequent premature solidification of the suspension in the flow channels. To avoid this, excess coating material is removed from the flow channels by being sucked or blown clear while still in the wet state. For this to proceed with the necessary reliability, the solids content and the viscosity of the coating suspension have to be matched to the suction capacity of the honeycomb body.
The mean layer thicknesses of the catalytic coating produced in this way are in the range between about 10 and at most 100 xcexcm, whereby considerable differences in layer thickness may exist over the cross-section of a flow channel as a result of the capillary forces which act during coating. Within the process tolerances, however, the mean layer thicknesses are identical over the entire cross-section of the honeycomb body, i.e. the mean layer thicknesses have a radially homogeneous distribution. By means of special measures during coating, radially inhomogeneous layer thickness distributions can be obtained. Thus DE 39 12 915 C1 describes honeycomb bodies with layer thicknesses of the catalytic coating which are greater in the centre of the honeycomb body than at the edge. This radial inhomogeneity is adjusted by using appropriate diaphragms when the honeycomb body is coated.
The ceramic honeycomb bodies predominantly used in exhaust gas catalysis at the present time are produced by extruding ceramic pastes. They have square or rectangular flow channels with cell densities (number of flow channels per unit cross-sectional area) of 62 cmxe2x88x922, the thickness of the channel walls being approx. 0.16 mm. The jacket delimiting the honeycomb body can have the same thickness as the channel walls. Usually, however, the jacket is designed somewhat thicker than the separating walls of the channels in order to increase the mechanical stability.
To improve the catalytic conversion of the pollutants, honeycomb bodies are being developed which have cell densities of up to 200 cmxe2x88x922 and wall thicknesses of only 0.1 mm or less. These high-cell honeycomb bodies provide an appreciably greater geometric surface for the catalytic coating and, by virtue of their smaller mass, they heat up to the operating temperature of the catalyst much more rapidly. Because of the small channel cross-sections, these high-cell honeycomb bodies tend to suffer from increased obstruction of the flow channels during coating. According to DE 198 10 260 A1, this tendency can be counteracted by wetting the entire honeycomb body evenly prior to coating, thereby saturating a substantial part of the suction capacity of the honeycomb body. However, compared with an unwetted honeycomb body, this causes an undesirable reduction in the achievable coating concentration when using an already highly concentrated coating suspension.
Because of the small wall thicknesses, the mechanical strength of the high-cell honeycomb bodies is lower than that of the conventional honeycomb bodies. To increase the mechanical strength, attempts are therefore made to provide the outer layers of flow channels, adjacent to the jacket, with larger wall thicknesses than those in the centre of the honeycomb bodies. Such honeycomb bodies provided with unequal wall thicknesses are referred to hereafter as xe2x80x98inhomogeneous honeycomb bodiesxe2x80x99 for short. They are described for example in DE 199 02 540 A1.
Coating of honeycomb bodies with the coating suspension is accompanied by the danger, already described, of obstruction of the flow channels by coating material, obstructed channels being observed principally in the marginal region of the honeycomb bodies. This applies particularly to the inhomogeneous honeycomb bodies described.
An object of the present invention is therefore to provide a coating process which reduces the risk of obstructed flow channels in the marginal region of honeycomb bodies, especially of inhomogeneous honeycomb bodies.
This and other objects can be achieved by a process for coating, with a suspension, a ceramic honeycomb body which has a cylindrical shape with first and second flat end-faces and a jacket, and through which channels parallel to the axis, formed by channel walls, run from one flat end-face to the other, the honeycomb body being coated by suitable processes. The process is characterized in that the honeycomb body is partially wetted and then coated.
Within the framework of the present invention, wetting is understood as meaning the covering of the porous honeycomb body with any desired liquids or solutions, preferably of an aqueous nature. Coating is always completed by drying and calcining the honeycomb body.
The invention is based on the knowledge that, when using an identical coating suspension, less coating mass can be deposited on wetted honeycomb bodies than on dry honeycomb bodies. If the honeycomb body is only partially wetted, as proposed, it is therefore possible to deposit greater amounts of coating and to avoid channel obstructions in sections of the honeycomb body which are critical due to the coating process.
The proposed process also allows inhomogeneous distributions of the coating concentration. The inhomogeneous distribution can be produced at any desired points, both over the length of the honeycomb body and over its cross-section. The degree of saturation of the suction capacity of the porous honeycomb body can be controlled by those skilled in the art by means of the liquid volume used and optionally by introducing a waiting time between wetting and coating. The degree of saturation determines the amount of coating suspension deposited and hence the coating concentration on the coated honeycomb body.
It is particularly advantageous to wet the jacket surface of the honeycomb body. It has in fact been found that the increased number of obstructed channels in the marginal region of the honeycomb bodies is caused by the greater accumulation of honeycomb body material due to the jacket, and the resulting increased suction capacity for the liquid phase of the suspension. This applies both to honeycomb bodies whose jacket has the same thickness as the channel walls, and to honeycomb bodies with a thicker jacket, and also especially to inhomogeneous honeycomb bodies with thicker channel walls in the region of their jacket.